This invention relates in general to magnetic brush development apparatus, and more particularly to a magnetic brush development apparatus of reduced size and which maintains substantially optimum pigmented marking particle (toner) concentration without the use of a toner monitor.
In electrostatographic reproduction apparatus, it is a well known practice to develop a latent image charge pattern on a dielectric member with pigmented marking particles by applying such particles to the image with a magnetic brush development apparatus. The typical magnetic brush development apparatus includes a sump containing a mixture of magnetic (ferrite) carrier particles and relatively smaller pigmented marking particles (commonly referred to as toner). The magnetic carrier particles and pigmented marking particles are agitated in the sump to effect a triboelectric attraction of the pigmented marking particles to the magnetic carrier particles. The magnetic carrier particles with the attracted pigmented marking particles are fed to a development roller which includes an alternating pole magnet within a nonmagnetic shell. The magnet and shell of the development roller rotate relative to one another causing the magnetic carrier particles (and attracted pigmented marking particles) to form a multibristle-like arrangement on the shell in the field of the magnet. The bristles, moving as the magnet and shell relatively rotate, are swept over the dielectric member in a development zone where the pigmented marking particles are attracted from the magnetic carrier particles to the latent image charge pattern to develop the pattern.
During operation of the magnetic brush development apparatus, the combined magnetic carrier particle/pigmented marking particle material is constantly fed to the development roller upstream of the development zone and removed from the roller downstream of the development zone (by a mechanical skive for example). A relatively large amount of magnetic carrier particles are required in the described circulation of material between the development apparatus sump and the magnetic brush roller to adequately effect latent image charge pattern development. This results in a development apparatus of substantial size. Moreover, since the pigmented marking particles are constantly being removed from the combined material in order to effect development, the concentration of pigmented marking particles (toner) in the combined material must be monitored to assure sufficient latent image charge pattern development without image disrupting defects or underdevelopment of the latent image charge pattern. Toner monitors and the associated control devices to regulate pigmented marking particle concentration require extremely sensitive complex instruments and apparatus. Such instruments and control apparatus are subject to numerous failure modes during which defective image development may occur, or in the extreme, may require that the development apparatus be shut down completely.